Glass wool is well recognized as an efficient insulating material in that wool of low density gives good insulation, that is to say a low Lambda (thermal conductivity) value.
Stone wool is formed from molten mineral such as one or more of diabase, basalt, slag, limestone, dolomite, cement, clay, feldspar, sand or olivin and generally contains stone wool another good insulator, but stone wool needs to be of a higher density than glass wool to achieve a similar Lambda value. Thus more material is required. This is commercially satisfactory for most purposes because it is cheaper to make stone wool than glass wool and so the stone wool is competitive with, or economically preferred over, the corresponding glass wool despite the necessity to have a higher density. Stone wool is cheaper because it is made from cruder, iron-containing melts that are not suitable for glass wool, and glass wool requires purer melt including expensive additives such as soda and borate in the melt.
Stone wool also has some properties which glass wool does not have, i.e. higher temperature resistance and better water repellancy, and better sound absorption, because of the higher density. Consequently stone wool can be used in areas where glass wool is less efficient.
It would of course be highly desirable to be able to reduce the density of the stone wool without decreasing its performance, since the stone wool would then be even more cost effective or preferred over glass wool, for a given degree of thermal insulation. Unfortunately this has not proved possible.
Because of the wish to make stone wool in a cost-effective manner, for it to be competitive with glass wool, the emphasis has always been on maximizing the amount of wool that can be obtained in any particular wool-forming apparatus, and it has been accepted that the wool will inevitably contain coarse fibers and significant amounts, e.g., over 25% by weight, of shot having particle size above 63 .mu.m.
The industry has accepted these restrictions as inevitable and has concentrated on ways of improving the productivity of processes for making such products.
In the following, the measurement of fibers diameter is referring to the standard accumulated length-based method carried out using a light microscope or a scanning electron microscope. The result on shot content is based on DIN standard No. 4188. The measurements on lambda are based on DIN No. 52612 part 1 and 2. Fiber length is based on 50% quantile on a logarithmic normal distributed graph. Density is calculated from measurement of volume according to DIN 18165 product type WL. The tensile strength measurements are referring to DIN No. 18165, measured on 30 kg/m.sup.3 cured product with a binder content of 1.4% by weight.